Truly depressed people (as mentioned in the article) will seek social isolation, so they will probably avoid a situation like participating in a study. However, I think they're just trying to show that feeling depressed will lead to better problem solving. So while the participants may not actually be suffering from depression, their depressed moods may be helping them with the math problems. On the other hand, there might be a peak at which feeling blue will help with problem solving. Chronic depression may elicit learned helplessness when it comes to problem solving which would be detrimental to the results.As for your other point, I agree, rumination and social isolation simply leads to more rumination and social isolation. It's a vicious circle: You feel alone, so you keep thinking about how terrible it is to be alone and the mistakes you had to make you like that. You start pushing people away from you because you want to reaffirm your loneliness (self-fulfilling prophesy), leading to more feelings of being alone. Even though you may know why you are feeling alone and thus depressed, say because you're afraid of rejection, it does not mean that the depression itself has any upside in helping you deal with the fear of rejection. If anything, it will condition you to be even more afraid of rejection due to the increased social isolation.